[Haskell-cafe] monadic plumbing
Holger Siegel
holgersiegel74 at yahoo.de
Tue Feb 22 22:31:12 CET 2011
Am 22.02.2011 um 22:03 schrieb Alberto G. Corona:
> Recently I had to navigatate trough data structures chained with mutable referenes in th STM monad. The problem is that their values are enveloped in Either or Maybe results.
>
> functional compositions in the Either of Maybe , or list monads are not possible when the values are embedded inside effect monads (i.e. STM or IO) . I tried to find some trick to handle it.
>
> to summarize, given:
>
> foo, : a -> m (Maybe b)
> bar : b -> m (Maybe c)
> baz : c -> m (Maybe d)
>
> how to compose foo bar and baz? Or, at least, Are something out there to handle it in the less painful way?.
import Control.Monad.Trans.Maybe
fooBarBaz = runMaybeT (MaybeT . foo >=> MaybeT . bar >=> MaybeT . baz)
(untested)
>
> I solved the generalized problem (chaining any double monadic combination) with a sort of monadic connector that acts as a " double monadic" operator >>>>== so that
>
> return. return (x :: a) >>>>== foo >>>== bar >>>== baz
>
> can be possible. Although I don't know if it is the best solution. I wonder why nobody has written about it before:
>
> class (Monad m, Monad n) => Bimonad m n where
> (>>>=) :: n a -> (a -> m(n b)) -> m(n b)
>
> (>>>>==) :: (Bimonad m n) => m (n a) -> (a -> m(n b)) -> m (n b)
> (>>>>==) x f = x >>= \y -> y >>>= f
>
> x >>>> f = x >>>>== \ _-> f
>
> infixl 1 >>>>==, >>>>
>
> The instance for handling the Maybe monad under any other monad is very similar to the definition of the "normal" monad:
>
> instance (Monad m) => Bimonad m Maybe where
> Just x >>>= f = f x
> Nothing >>>= _ = return $ Nothing
Ignoring the newtype wrappers, this is the same as the actual monad instance of MaybeT:
newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) }
instance (Monad m) => Monad (MaybeT m) where
fail _ = MaybeT (return Nothing)
return = lift . return
x >>= f = MaybeT $ do
v <- runMaybeT x
case v of
Nothing -> return Nothing
Just y -> runMaybeT (f y)
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